voltage drop/capacity for an off-grid system?

spencex · June 2011

I'm using 6 flooded Trojan T105 batteries on a 12v system with 675 aH of capacity with two Blue Sky 2512ix controllers and a proremote display with approximately 600 watts of panels. The batteries are less than a year old, having been manufactured last October, fully charged last January and placed in service in March. I equalized them last week (wasn't able to before).

The primary loads are an electric refrigerator using 10-15 amps and a small TV/satellite receiver using about 10 amps.

At the end of the solar day, I've noticed that there is a relatively fast drop in voltage down to 12.6v then to 12.4v and then 12.3v. It usually hovers around 12.2v or 12.3v in the evening. The next morning, while under load from the refrigerator, the voltage will typically read 11.9v-12v, while the display will say there is 85% remaining battery capacity. Are these normal drops in voltage?

Doing Trojan's recommended 6-hour no-charge or discharge test of fully charged batteries, the weakest battery shows a charge of 6.31v, the next weakest 6.32v, followed by 6.34v and the remaining at 6.35v.

Now I'm wondering is the weakest batteries are the problem or if perhaps the controllers are not fully charging my battery bank--or if everything is normal and I'm worrying too much. I want to take good care of my expensive batteries.

Would appreciate any help or suggestions.

niel · June 2011

Re: voltage drop/capacity for an off-grid system?

welcome spence,
this may not be a battery problem yet as it may be your wires with possible weak connections. how you have the batteries interconnected could also cause some minor discrepancies so how do you have them wired and with what gauge #?
this may help you as well;
http://www.smartgauge.co.uk/batt_con.html

Cariboocoot · June 2011

Re: voltage drop/capacity for an off-grid system?

Welcome to the forum.

Let's look at what you should have to get Trojan's recommended 10% charge rate @ 14.8 Volts:

67.5 Amps * 14.8 Volts = 991.6 Watts, less typical derating factor = 1240 Watt array.

So your array is about half of what the battery manufacturer would want. That's a problem. Right now your recharge rate is less than the 5% minimum and far less than Trojan's 10% recommendation. They probably do not spend enough time in Absorb as the smaller array won't bring the charge up quickly enough.

Some other potential trouble makers for this system: wire sizes. 12 Volt systems are particularly sensitive to having the right wire size, and that depends on the expected current flow and the length of the wires. Keeping the cables from the batteries to the inverter as short and large as possible is a must.

Uneven current through the batteries. It's difficult to get this right with three parallel battery banks. One set may not be contributing to output while at the same time getting plenty of recharge.

Suggestions: take one set of batteries out of the equation so that you have 450 Amp hours @ 12 Volts. Connect the inverter and charge controller (-) to one string, the (+) to the other. This is "diagonal wiring" and is good for forcing current through both strings.

Get a hydrometer and check the Specific Gravity of each cell. There's no better way to tell what state the battery is in.

Get a Kill-A-Watt meter and get some specific daily consumption figures in Watt hours. You need this to properly design a system.

BTW, I run pretty much the same equipment on a 24 Volt system with 320 Amp hours and 700 Watts of panel. It's undersized and necessitates some serious load management.

niel · June 2011

Re: voltage drop/capacity for an off-grid system?

good catch coot as he is borderline even for on our usual 5%-13% range. i still think it's a problem concerning the wire arrangement, wire size, or connections, but it would be a good idea to up the pv power some for sure.

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

Thanks for the replies.

The cables connecting the 3 sets of batteries are 4 gauge and as short as possible. The cables connecting batteries to the inverter are "welding cables" and appear to be about twice the size of the 4 gauge cables. The wires connecting the PV array to the Blue Sky controllers are with Blue Sky specifications for length (8 or 10 gauge or better). The connections are good.

I will remove one set of the batteries and see what happens.

I find it curious, though, that my proremote display tells me that my 3 sets of batteries are fully charged--usually by 1 pm or so.

Cariboocoot · June 2011

Re: voltage drop/capacity for an off-grid system?

I'm not sure how good the BlueSky equipment is at judging charge. Most charge controllers go by Voltage only, and having reached a "target" Voltage is not the same as having recharged the batteries.

Have a good read through the battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

There's nothing like a hydrometer for telling what state the batteries are really in. And even that can't tell you about the condition of the plates inside.

You will lose capacity by removing one set of batteries, but the remaining two may charge faster and more completely. If you can get them recharged early in the day so that they are in Float, then run loads without dropping out of float and save your battery capacity for necessary night use you might actually gain.

But if your total daily loads exceed the Watt hours the panels can produce in a day you will have deficit charging no matter what the rate, and that's not good.

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

The manual for the IPN Pro Remote monitor and the Solar Boost 2512iX controller are available from Blue Sky's website here if you're curious:

http://www.blueskyenergyinc.com/manuals/

Tomorrow I'll post the result of removing one pair of batteries.

Thanks again.

mike95490 · June 2011

Re: voltage drop/capacity for an off-grid system?

And there is my favorite trick, if you have a genset, and a battery charger, fire the genset up for half / full hour in the AM, and bulk charge the batteries with a charger, and then let the solar finish them up. 2 or 3 days of that should get them full.

I don't see how, with your low array size, you could have really equalized them the other day.

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

How much amperage do you pump into the battery bank? I could do 2, 6, 8 or 35 amps.

According to my monitor, the equalization succeeded--although it took all day, on and off--to reach two hours total at 15.5 volts.

rplarry · June 2011

Re: voltage drop/capacity for an off-grid system?

spencex
Battery monitors are not a very good way to tell when a battery is fully charged. Specific gravity is a good way, but you have to factor in battery temp. Another good way to determine when you bank is fully charged is to use the "amps in" portion of your monitor and see when the amps going into the bank tapers down to 1% of the amphour capacity of your bank. So in your case with all 6 batteries you would be fully charged when the charge tapered down to 6 or 7 amps. At that point your batteries won't accept any more charge and you can consider them fully charged. You might have to run your gen for an hour or 2 in the morn and let the panels top it off to get to that 1% mark.
Here is a link to Trojans battery maintaince page, lots of good info on how to charge and maintain their batteries.
http://www.trojanbattery.com/Tech-Support/BatteryMaintenance.aspx
Good luck with yours,
Larry

niel · June 2011

Re: voltage drop/capacity for an off-grid system?

spencex wrote: »

Thanks for the replies.

The cables connecting the 3 sets of batteries are 4 gauge and as short as possible. The cables connecting batteries to the inverter are "welding cables" and appear to be about twice the size of the 4 gauge cables. The wires connecting the PV array to the Blue Sky controllers are with Blue Sky specifications for length (8 or 10 gauge or better). The connections are good.

I will remove one set of the batteries and see what happens.

I find it curious, though, that my proremote display tells me that my 3 sets of batteries are fully charged--usually by 1 pm or so.

you could still have a problem with the wires if the lengths of the wires are too long for the current being carried. this is not only true going from the pvs to the controller, but also the controller to the batteries is critical as any voltage drops there could fool the controller into thinking the batteries have reached their full charge. you can check the vdrop by going to the vdrop calculator seen in my signature line. also be sure that the wires are not differing lengths that could cause small resistance fluctuations from one bank to another. these wires for the battery bank interconnections should be much thicker than the #4 you use everywhere else. i use #2/0 (that's #00) for my 400ah battery bank and you should probably have a gauge number like this or better. even on the lite side of things it should not be less than #0 imho.

i am not familiar with the remote you cited and i didn't read up on it, but unless the cc can compensate for the voltage drops introduced by the wires the cc will see a higher voltage than that which is at the batteries due to the vdrop of the wires. that could prevent the proper charging of the batteries and if they aren't getting their full charge then this could lead to further problems. now this may not be as big of a problem as i make it sound, but it is something you should be aware of. you are currently feeding a smaller charge current and the impact of vdrop won't be as bad with smaller currents. verify that when you see a full charge indicated that the voltage on the batteries themselves measured with a good dmm or dvm can show that the voltage is proper on the batteries. note also if they may vary in voltage even slightly as that could be due to vdrops and is worth investigating further.

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

The default setting for acceptance mode with my BS 2512iX controllers is 14.2v. I'm wondering if I should up that to 14.8v since it appears my batteries are not receiving that much voltage except in equalization mode.

Okay, here's what's going on with the two pairs of Trojans (450 aH) set up and the third pair idle:

At the end of the solar day Saturday, my battery monitor was showing 99% remaining battery capacity, so I topped the battery bank up with my generator charging at 8 amps. I then ran the generator to power the house until around 8:30 pm when voltage was showing 12.6 and the batteries/inverter took over.

At 6 am this morning (Sunday), the monitor showed 85% remaining battery capacity, and the voltage was 12.1 with a light 2 amp load. When the fridge cycled on, the voltage dropped to 11.9.

I ran the generator for about an hour or so charging the batteries at 8 amps and also powering the house.

At 7:30 am, with only a few amps coming in from the sun, I let the batteries/inverter/solar panels take over.

At 10:30 am, the controllers go to acceptance mode, the monitor shows 90% remaining battery capacity and the monitor shows about 25 amps arriving at the batteries.

At 11:30 am, the controllers are still in acceptance mode but I'm now seeing 94% and 25 amps are coming in (29 aH away from full).

At 12:30 pm, float has been reached, remaining battery capacity is 100%, 7 amps are coming in (1 aH from full).

Can anyone make any sense of what is going on here? Is the problem that 14.8 volts doesn't appear to be getting to the batteries? Again, should I increase the acceptance voltage? According to the monitor, there is 19v available at times.

Thanks!

niel · June 2011

Re: voltage drop/capacity for an off-grid system?

put the acceptance to 14.8v as your batteries are being undercharged. do this now and i hope you did not cause sulfation to occur in the batteries. the other points brought up in this thread are valid too as the current levels are too low, but you did a major mistake putting the battery voltage down to 14.2v on trojans.

note that the charge current with loads on will have the net charge current be the current you supply minus the load current and that further deprives the batteries.

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

niel wrote: »

you could still have a problem with the wires if the lengths of the wires are too long for the current being carried. this is not only true going from the pvs to the controller, but also the controller to the batteries is critical as any voltage drops there could fool the controller into thinking the batteries have reached their full charge. you can check the vdrop by going to the vdrop calculator seen in my signature line. also be sure that the wires are not differing lengths that could cause small resistance fluctuations from one bank to another. these wires for the battery bank interconnections should be much thicker than the #4 you use everywhere else. i use #2/0 (that's #00) for my 400ah battery bank and you should probably have a gauge number like this or better. even on the lite side of things it should not be less than #0 imho.

i am not familiar with the remote you cited and i didn't read up on it, but unless the cc can compensate for the voltage drops introduced by the wires the cc will see a higher voltage than that which is at the batteries due to the vdrop of the wires. that could prevent the proper charging of the batteries and if they aren't getting their full charge then this could lead to further problems. now this may not be as big of a problem as i make it sound, but it is something you should be aware of. you are currently feeding a smaller charge current and the impact of vdrop won't be as bad with smaller currents. verify that when you see a full charge indicated that the voltage on the batteries themselves measured with a good dmm or dvm can show that the voltage is proper on the batteries. note also if they may vary in voltage even slightly as that could be due to vdrops and is worth investigating further.

You might be onto something, Niel. I'll get bigger cables for the batteries and double check the AWG for the wires going to the controllers and from the controllers to the shunt/batteries.

Right now, at float, the monitor is showing 13v while my DVM is saying 12.8v. Not much of a difference, but I have seen differences of as much as 1v and wondered why.

Thanks for the suggestions from you and everyone else! If I can get to the bottom of this, I will report.

Spence

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

mike90045 wrote: »

And there is my favorite trick, if you have a genset, and a battery charger, fire the genset up for half / full hour in the AM, and bulk charge the batteries with a charger, and then let the solar finish them up. 2 or 3 days of that should get them full.

I don't see how, with your low array size, you could have really equalized them the other day.

Mike, you might be correct that equalization did not occur. It appears that the controller thinks that higher voltage is at the batteries than there actually is. For example, I ran equalization this afternoon and while the battery monitor was indicating 15.5v, my voltmeter at the battery bank showed less than 15v.

I'm going to do some rewiring as soon as possible with heaver gauge materials.

Thanks for your observation!

Spence

spencex · June 2011

Re: voltage drop/capacity for an off-grid system?

To all who replied with suggestions, many thanks! Your comments worked.

The problem with my setup was my ignorance. I did not know how important sufficient wiring sizes are with 12v systems. Upping wire guage where necessary did the trick.

Now, my 675 amp battery bank gets fully charged during the day with my 680 watt PV array. In the morning, I show around 12.3 volts and 80 percent or so remaining battery capacity. Sounds about right to me.

I am, however, planning to add 130 watts this fall to help compensate for the shorter days and to help provide more voltage for equalization.

You folks are great!

Spence

voltage drop/capacity for an off-grid system?

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